Why is a calcium atom larger than a magnesium atom?

Question

Please help explain in terms of not just periodic trends.

Answer 1

Hi.

An atom's size depends, to a significant degree, on the number of electrons it has. Because a calcium atom has more electrons (20) than a magnesium atom does (12), some of its electrons will be farther from the nucleus in a higher energy level.

We can envision this by taking a look at these two elements' electron configuration:

₁₂Mg: 1s²2s²2p⁶3s²

₂₀Ca: 1s²2s²2p⁶3s²3p⁶4s²

The magnesium atom's electrons (12e-) occupy fewer energy levels than do the electrons of the calcium atom (20e-).